In the prior art, thread wound golf balls are generally manufactured by winding a rubber thread with a high elongation on a core of rubber or liquid to form a core ball, and enclosing the core ball in a cover. For winding thread rubber around the core, there are known twc techniques, a random winding or basket winding technique and a great circle winding technique.
The random winding technique is disclosed in Japanese Patent Application Kokai Nos. 15363/1985 and 15364/1985 and may be carried out with an apparatus as shown in FIG. 1. The winding apparatus 10 of FIG. 1 includes a pair of drive rollers 12 and 14 on which a core 16 rests. The winding apparatus 10 further includes a pneumatic cylinder 24 coupled to a support 26 which bears for rotation idle rollers 20 and 22 in press contact with the core 16. The drive rollers 12 and 14 cooperate to rotate the core 16 while the drive rollers alternatively move back and forth in an axial direction. While the core 16 randomly rotates in this way, a rubber thread 18 is wound on the core 16 through the idle rollers 20 and 22. The pneumatic cylinder 24 is periodically actuated to reduce the pressing force of the idle rollers 20 and 22 against the core 16, making motion of the core 16 free from the drive rollers 12 and 14. This allows the core 16 to spontaneously move such that the rubber thread 18 is wound with where it has been wound less turns. In this way, the rubber thread 18 is eventually wound in a spherical form as a whole. Although the rubber thread orbits have a basic pattern of sine wave.like curves drawn on a sphere, periodic release of the core 16 from the drive rollers 12 and 14 induces a periodic change in the orbital curves, eventually providing random orbits.
The great circle winding technique is disclosed in Japanese Patent Application Kokai No. 211275/1986 and may be carried out with an apparatus as shown in FIGS. 2A and 2B. The winding apparatus 30 of FIGS. 2A and 2B includes a center-tapered drive roller 32, a center-bulged idle roller 34, and a holding idle roller 36 arranged at apexes of a triangle for supporting a core 40. Rotation of the drive roller 32 causes the core 40 to rotate whereby a rubber thread 42 is wound on the core 40 through the drive roller 42. Since only the drive roller 32 imparts rotation to the core 40, the rubber thread 42 is wound on the core 40 along its great circle. Because of the configuration and arrangement of the rollers, every turn of rubber thread 42 has its orbit moved slightly off the previous orbit. As a consequence, the rubber thread 42 is wound along orbits spreading like a fan shape as shown in FIG. 3. The great circle winding technique has an advantage that the winding apparatus is simple.
The thread wound golf balls are manufactured by winding thread rubber on a core by either of the above.mentioned techniques to form a core ball and then enclosing the core ball with a cover as by press molding. Conventional thread wound golf balls have a drawback that the covers are ruptured and the balls are deformed by hitting because they are less durable against impact.
The bond between the cover and the thread rubber layer is one of the factors which contributes to the hitting durability of thread wound golf balls. In addition to the fact that the durability of the cover material and thread rubber themselves is important, the cover tends to be readily ruptured if it is not in tight bond to the underlying thread rubber layer. To enhance the bond between the cover and the thread rubber layer, it is critical how the covering material well penetrates through the thread rubber layer during heat press molding of the cover. The penetration of the covering material is generally promoted by increasing the heating temperature and/or time during heat press molding. An increased heating temperature and an extended heating/pressing time assist in the penetration of the covering material, but cause the thread rubber to deteriorate and the ball to lose its hardness and performance.